Our overall objectives of the project is to delineate the role of reproductive hormones in the induction and development of spontaneous mammary tumors (SMT) in aging female rats. We plan to correlate the changes in reproductive patterns (and reproductive hormones) with the incidence and growth of spontaneous mammary tumor in order to study the hormonal responsiveness of these tumors. We have studied the hormonal responsiveness of a population of aging female rats bearing spontaneous mammary tumors. Following ovariectomy of tumor bearing rats, the majority of the SMT in these rats exhibited pronounced decrease in tumor size and subsequent regrowth with a lag period of 1 to 2 months, suggesting hormonal dependency of SMT. A second population of 50 aging female rats has also been maintained to study the effect of hormonal ablation therapies. Histological examination indicated that most of SMT studied are adenofibroma exhibiting varying degree of secretory activity. In search of a tumor marker for monitoring the possible hormonal regulation of tumor growth, we measured alpha-LA (B protein of lactose synthetase) content in the cytosol fraction (10,000 x q of SMT by a specific radioimmunoassay procedure employing staphylococcus A to precipitate antigen-antibody complexes. High concentration of alpha-LA was found in mammary tissues of lactating rats (21.9 plus or minus 5.6 microgram/mg protein; mean plus or minus S. E., n equals 6), whereas no alpha-LA was detected in uterus, kidney, fat, heart muscle thymus, and ovary, indicating the tissue specificity of alpha-LA production. In contrast, all 12 SMT examined exhibited varying but significant amounts of alpha-LA. Alpha-LA concentration is greater than 0.1% of the total protein in 66% of the SMT, whereas the remaining SMT have less than 1 microgram-alpha-LA/mg protein. These results demonstrate the substantial elevation of alpha-LA in SMT of aging female rats and suggest that alpha-LA may serve as a specific marker for monitoring the hormonal regulation of spontaneous mammary tumorigenesis.